1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and more particularly, to a zoom lens having a wide angle, high magnification, small size, and light weight, which is suitable for a broadcasting television camera, a video camera, a digital still camera, a silver halide film camera, and the like.
2. Related Background Art
In recent years, a zoom lens having a wide field angle, a high zoom ratio, and high optical performance is required for an image pickup apparatus such as a television camera, a silver halide film camera, a digital camera, or a video camera. As a zoom lens having a wide field angle and a high zoom ratio, there is known a positive lead type five-unit zoom lens constituted of five lens units including a positive refractive power lens unit disposed closest to the object side. As this positive lead type zoom lens, there is known the five-unit zoom lens that is particularly suitable for a television camera, in which a zooming lens unit having functions as a variator and a compensator is constituted of three movable lens units, which move in different loci (see U.S. Pat. Nos. 4,874,231 and 5,808,809).
U.S. Pat. No. 4,874,231 proposes a zoom lens constituted of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power for zooming, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a positive refractive power for imaging. In addition, U.S. Pat. No. 5,808,809 proposes a five-unit zoom lens in which a second lens unit for zooming (variator) is constituted of a second A lens unit having a negative refractive power and a second B lens unit having a positive refractive power, which move in different loci during zooming.
Further, various types of so-called rear-focus zoom lenses in which focusing is performed by a lens unit other than the first lens unit are proposed. Japanese Patent Application Laid-Open No. 2005-292605, for example, proposes a lens device which includes a first lens unit which has a positive refractive power, a second lens unit for zooming which has a negative refractive power, a third lens unit for image plane compensation, and a fourth lens unit for imaging which has a positive refractive power, in which a focusing operation is performed by the fourth lens unit.
In order to obtain high optical performance while maintaining wide field angle and high zoom ratio in the five-unit zoom lens, it is important to set appropriately refractive powers of the second, third, and fourth lens units as zooming lens units, movement conditions during zooming, and the like.
In particular, it is important to set appropriately an imaging magnification and a moving locus of the second lens unit for zooming, and a moving locus of the third lens unit from the wide angle end to an intermediate zoom position.
If such structures are not set appropriately, it is difficult to realize a small size of the entire system and to obtain a zoom lens having a wide field angle, a high zoom ratio, and high optical performance over the entire zoom range.
Further, in a conventional four-unit zoom lens for a television camera, when even wider angle and higher magnification are to be achieved with reductions in size and weight, the refractive power of each unit needs to be increased, which results in a problem of larger variations of various aberrations.
In U.S. Pat. No. 4,874,231, when zooming is performed from the wide angle end to the telephoto end, two zooming units move in different loci to thereby compensate optical performance appropriately at the intermediate zoom position, but reductions in size and weight are not achieved. Further, in the conventional four-unit zoom lens for a television camera, when the reductions in size and weight are to be achieved for the first lens unit, it is necessary to reduce the number of lenses in the first lens unit or to increase the refractive power, which results in difficulty in suppressing variations of various aberrations due to focusing.
In Japanese Patent Application Laid-Open No. 2005-292605, the rear-focus method is employed to achieve the reductions in size and weight of the first lens unit. However, generally in the zoom lens for broadcasting, an optical system for converting a focal length is detachably disposed inside the fourth lens unit for imaging, which poses a problem that the movement amount accompanying focusing on the telephoto side at an object distance of proximity is increased when the optical system for converting a focal length is attached.